This invention relates to the art of waveguides and, more particularly, to providing a transition or coupling between two waveguides such as a circular wrapped rectangular waveguide and a rectangular waveguide.
Slotted cylindrical antennas have been used effectively in UHF broadcasting. One such antenna is a slotted length of circular wrapped rectangular waveguide which, in cross section, has a lunar appearance. Such a lunar waveguide is similar to that described in the prior art, such as in U.S. Pat. Nos. to L. J. Chu 2,477,510 and to S. A. Schelkunoff 2,199,083.
Such a lunar waveguide may be considered as an elongated circular wrapped rectangular waveguide having an elongated metallic annular wall having an upper end and a lower end. A partition member, known as a septum, extends radially inward from the annular wall. The inner edge of the septum may be rounded somewhat and is referred to as a ridge. This ridge is located near the central axis of the annular wall and extends along with the septum longitudinally for a length corresponding with that of the annular wall.
The input to such a lunar waveguide has typically taken the form of a coaxial input. One type of a coaxial input known in the art has included in a bottom fed construction wherein the inner conductor of the coaxial input was connected to the ridge and the outer conductor was connected to the annular wall of the lunar waveguide. Another form of coupling power to such a lunar waveguide has included a coaxial input mounted on the side of the waveguide. The inner conductor of the coaxial input extends through the annular wall of the waveguide and makes electrical connection with the ridge on the septum and the outer conductor of the coaxial input is electrically connected to the annular wall.
Problems have been noted with such connections of coaxial inputs to a lunar waveguide antenna, presenting a need to eliminate such coaxial connections. One of the problems noted is that the input power handling capability of such a waveguide antenna is much greater than the power handling capability of the coaxial input, thereby limiting the capability of the waveguide antenna to that of the coaxial input. If too much power is supplied to the coaxial cable, it will overheat and burn up. There are limitations in the diameter of such a coaxial input, as it is difficult to obtain a proper impedance match between such a coaxial connection and a transmitter due to higher order modes present in the coaxial cable.
Many high power UHF stations have found it desirable to utilize rectangular waveguide transmission lines to increase system efficiency in transmitting power from the transmitting equipment to the waveguide antenna. In order to remove the power handling restrictions encountered with the coaxial to lunar transition, it would be desirable to provide a rectangular waveguide to a lunar waveguide transition. This will eliminate the need for a coaxial section and thereby provide a pure waveguide system.